SoCalLawGuy:
As far as I know, you have to register for the Bar Exam as soon as the first semester of law school (meaning your first year). If you miss that, you might end up paying late registration fees, but not all states have this rule.

SaraJean:
Actually, it depends on the jurisdiction. Your state supreme court's website should have the deadlines. The administrators at your school should also be able to answer your questions.

In my state, law students must register as a "Candidate for Admission" by November 15 of their second year. This involves a very intense background check and an in-person interview with a member of the admissions committee. Then, 3Ls must register by April 1 if they wish to take July bar exam.